Photosynthesis• 6 CO2 + 12 H2O --> C6H12O6 + 6 O2 + 6 H2O
• carried out by photoautotrophs• Solar energy --> chemical energy
• redox process- water oxidized, CO2 reduced
• Photosynthesis occurs in 2 stages:• 1. light reactions (photo)• 2. Calvin cycle (synthesis)
• Chloroplasts• found in mesophyll• contain chlorophyll = green pigment• thylakoid membranes in stacks called grana• in prokaryotes, thylakoids on cell membrane
• In each chloroplast are functional groups called
photosynthetic units• 1. contain about 300 molecules of
pigment• 2. contain one specialized chlorophyll a
molecule called the reaction center• 3. other molecules function like antennae• 4. reaction center can trap energy and
pass it along an enzyme-linked series of reactions to convert energy to a
usable form
• chlorophyll a- • 1. absorbs violet, blue-violet, red• 2. reflects green, yellow, orange
• carotenes- • 1. absorb wavelengths not absorbed by
chlorophyll• 2. provide photoprotection
•**All pigments absorb different wavelengths!**
• electrons within chlorophyll are excited to higher energy levels by photons (particle of light)
• What happens to excited electron?• 1. energy dissipated as heat• 2. energy reemitted as light
=fluorescence (in test tube)• 3. energy may cause a chemical
reaction (in chloroplast)
1. Light Reactions
• Light energy drives the transfer of e- and H from water to NADP+ ---> NADPH
• H2O is split and O2 is given off.
• Light reactions also carry out photophosphorylation converting ADP to ATP.
• Photosystem I= P700
• Photosystem II= P680
• Have identical chlorophyll a but different associated proteins
Noncyclic Photophosphorylation
• •occurs in green plants and a few bacteria• 1. Photon strikes P680 reaction center.• 2. Electron passed down ETC to P700.• 3. Some energy in chain used to make ATP
by chemiosmosis across the thylakoid membrane.
• 4. Electrons in P680 replaced by splitting H2O.• ** This is the step where O2 is released!**
• 5. Photon strikes P700 reaction center.• 6. Electrons passed through a second ETC.• 7. More ATP is made by chemiosmosis.• 7. As e- pass down ETC, NADP+ is reduced ->
NADPH.• **NADPH and ATP go on to the Calvin cycle.**
Cyclic Photophosphorylation
• •less efficient
• •only uses P700
• 1. Photon strikes P700 reaction center.
• 2. Electrons passed down ETC back to P700.
• 3. Generates ATP.
• 4. No production of NADPH or release of O2.
• ***Like oxidative phosphorylation in the mitochondria!***
• Cyclic Photophosphorylation continued…
• May have been the earliest form of ATP production.
• Still used by photosynthetic bacteria.
• Operates along with noncyclic flow in plants to generate more ATP. (The Calvin Cycle uses more ATP than is produced by
noncyclic photophosphorylation.)
2. Calvin Cycle
• •Doesn’t require light directly• (occurs during daylight for most plants!)
• Begins with Carbon fixation= Carbon from CO2 is incorporated into organic molecules
• 3 CO2 enter per cycle
• •The Calvin Cycle then reduces the carbon into carbohydrate by the addition of e- from NADPH
• •produces one 3C sugar (PGAL or G3P) per cycle
• Process: (for each CO2)• 1. CO2 attached to a five C sugar (RuBP)• -catalyzed by enzyme rubisco • (most abundant protein on Earth!!!)
• 2. Forms unstable 6-C intermediate which splits into 2 3-C sugars.
• 3. 3-C sugars phosphorylated by ATP.
• 4. 3-C sugars reduced by NADPH.
• 5. PGAL produced.
• 6 PGALs are produced for each Calvin Cycle• -1 exits cycle• -5 remain in cycle---> regenerate RuBP
• 6. Rest of cycle= regenerate RuBP (ATP needed)
• It takes 2 cycles to produce one glucose!
Special Cases of Photosynthesis
• Most plants= C3 plants
• CO2 fixed by rubisco & first product = 3C sugar
• In dry, hot weather, plants close stomata to reduce transpiration --> Reduces CO2 intake.
• Rubisco binds to O2 when [CO2] is low.
• =photorespiration- decreases photosynthesis
• C4 plants- form a four C compound as first product• ex- sugar cane, corn & some grasses• Krantz Anatomy- 2 types of photosynthetic cells• a. bundle sheath cells tightly packed around
veins• b. mesophyll cells located outside bundle
sheath• CO2 is fixed in the mesophyll by the enzyme
PEP carboxylase --> 4C compound • 4C compound then enters bundle sheath where
CO2 is released and is used by rubisco in the Calvin cycle.
• ***C4 pathway minimizes photorespiration and enhances sugar production!***
• CAM plants- no special cells, CO2 is trapped during the night (stoma open) for use during the day(stoma closed)
• incorporate CO2 into organic acids in night
• ex- cactus & pineapple
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